Add documentation that the sprig template library is supported in templates.

[mvanderlee]

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• I could not find a solution in the existing issues, docs, nor discussions.
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Ory Network Project

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Describe your problem

Please add documentation that the sprig template library is supported in the template syntax.
For a long time I've been manually trying to render a JSON string in the id_token claims field but was stumped on trying to handle nested fields. All I needed to do in the end was {{ toJson .Extra }}

After hours of trial and error I came across this file: https://github.com/ory/oathkeeper/blob/master/x/template.go which is how I discovered sprig. I only got to this particular file because I saw printIndex used in the documentation but didn't see it in the text/template's standard function documentation.
Perhaps sprig is a standard tool in the a Go developer's toolbox, but not being a Go developer myself I had no knowledge of this library.

My use-case is that I need to pass my custom hydrator's response into the JWT token.

Describe your ideal solution

It would be great if the use of sprig could be called out with some basic examples. For example, in the id_token documentation with toJson function usage.

Workarounds or alternatives

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Version

0.40.6

Additional Context

No response

[aeneasr]

Contributions highly appreciated! You know best where you're missing this info :)

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